Zircon Geochronology,Petrogenesis And Tectonic Setting Of The Triassic Intrusive Rocks From The Guokeshan Area,Northern Margin Of Qaidam Basin

Mafic microgranular enclaves(MMEs)are ubiquitous throughout the quartz-diorites in the Guokeshan area,northern magarin of the Qiadam basin.LA-ICP-MS zircon U-Pb dating gave identical ages of 247 Ma and 250 Ma for the host quartz-diorites and MMEs,respectively,which indicate that they are products of the Early to Middle magmatism along the northern magarin of the Qiadam basin.The host rocks are metaluminous to weakly peraluminous with A/CNK values of 0.95 to 1.02,indicating an I-type granite affinity.They are featured by enriched in LILEs and LREEs but depleted in HFSEs and HFSEs,which are typical features of arc-magmatic rocks.Their relatively flat HREE patterns imply that amphibole is the primarily mineral in residual phase.Comparision with petrology experimental suggests that they are likely products of partial melting of low-K basaltic rocks(amphibolite)rather than medium to high-K basaltic rocks,tonalites,and meta-greywackes.Additionally,the higher MgO contents and Mg# values compared with melts generated by partial melting of metabasalts/pelites under 1 to 4 GPa,suggest the involvement of mantle-derived materials into the sources of the quartz-diorites.The low Ti-in-zircon temperatures and high zircon Ce4+/Ce3+ ratios reveal that the quartz-diorites crystallized from low temperature and high oxygen fugacity magmas with incorporation of hydrous melts/fluids.Moreover,the quartz-diorites exhibit high La/Nb ratios of 2.27 to 2.64,consistent with the igneous rocks from active continental margin(>2.0),and fall within the field of volcanic arc granite in the granite tectonic environment discrimination diagrams.The first episode quartz-diorites were probably formed by mixing between felsic magms generated by partial melting of lower crustal amphibolites and mantle-derived mafic magmas under an active continental margin.The MMEs are dioritic in composition and show sharp contacts with their hosts.The enclaves are oval,lenticular and irregular in shapes,and occasionally they are weakly elongate and partially enclose back veins.Moreover,they exhibit clearly magmatic texture but are noticeably finer than the host in grain size.The disequilibrium textures,such as amphibole microcrystals enclosed within plagioclase,and the presence of hort column or acicular apatite in the MMEs,suggesting their derivation by magma mixing.In combation with their Nb/Ta and Zr/Hf ratios that were identical to the continental crust(Nb/Ta=11,Zr/Hf=33),we concluded that the MMEs were produced by mafic magmas injecting into and then incompletely mixing with felsic magmas.Combined with regionally tectonic evolution,we proposed that both the quartz-diorites and MMEs from the Guokeshan area in the Northern margin of the Qaidam basin were formed in an active continental margin generated by the northern subduction of Palaeo-Tethys ocean.